1. Field of the Invention
The present invention relates to an electrofusion joint for connecting a branch pipe with a main pipe made of a thermoplastic resin, particularly to an electrofusion joint provided with a saddle portion such as a saddle joint or service tee joint, to be joined at the outer circumferential surface of a main pipe.
2. Description of the Related Art
FIG. 5 is a side view of a saddle joint comprising saddle portion 1 and spigot 2 to which a branch pipe is to be connected, and which is generally made of a mold of medium density or high density polyethylene. As shown in the drawing, for electrofusion welding with a main pipe 3 made of a thermoplastic resin, the seating surface of the saddle portion is placed on the outer circumferential surface of the main pipe, and after clamping collar portion 4 on both sides of the saddle portion by engaging a latch of a clamping device that is not shown provided at the opposite side (lower side of FIG. 5), electric current is applied to heating wires embedded in the seating surface of the saddle portion.
During an electrofusion welding process, the seating surface of the saddle portion must remain in close contact with the outer circumferential surface of the main pipe. Otherwise, bubbles may be formed in the fusion welding interface during the electrofusion welding process, or the heating wires may move and contact each other to cause a short circuit.
However, since the joint is a molded product, it is difficult to mold it in such a manner that the seating surface of the saddle portion can achieve close contact with outer circumferential surface of the main pipe, and even if such precise molding is accomplished it will be difficult to fit the saddle portion on the main pipe by pressing the joint onto the main pipe from a lateral direction, if rigidity of the saddle portion is too large.
Wall thickness d of the saddle portion is generally designed to be around 10 mm regardless of the dimensions of the saddle portion, based on conventional usage. When wall thickness d is reduced, the saddle portion becomes less rigid and easier to fit to the main pipe. Therefore, a closer contact with the main pipe will be achieved. However, on the other hand, heat generated from heating wires through electric conduction is transferred to the outer surface if the saddle portion, and as a result the saddle portion tends to deform, resulting in lowered interface pressure and consequently in reduced fusion welding strength.
Therefore, as a general remedy a wall thickness d of approximately 10 mm is adopted for the saddle portion, and the saddle portion is made with a seating surface that has a slightly larger radius of curvature than the outer diameter of the main pipe, so that the saddle portion can be bent to make close contact with the lateral portions of the circumferential surface of the main pipe when clamped by a clamping device. On one side, if rigidity of the saddle portion is large, it can hardly be bent by clamping, and if the diameter of the spigot or trunk portion projecting from the saddle portion is large and the distance from the spigot or trunk portion to the collar portion is relatively short, the extent of possible flexure of the collar portion will be limited, resulting in inferior contact closeness with the outer circumferential surface of the main pipe.